U.S. patent number 4,163,702 [Application Number 05/891,438] was granted by the patent office on 1979-08-07 for process for rendering surfaces permanently water wettable and novel product thus-produced.
This patent grant is currently assigned to General Electric Company. Invention is credited to Milton E. Rickert, Jr..
United States Patent |
4,163,702 |
Rickert, Jr. |
August 7, 1979 |
Process for rendering surfaces permanently water wettable and novel
product thus-produced
Abstract
The surfaces of articles of manufacture fabricated from aluminum
and other material which are not permanently water wettable are
rendered permanently water wettable by coating the surface with a
continuous film of the free acid form of an acidic film forming
polymer which forms water soluble salts, said film containing a
curing agent for the polymer; contacting the polymer film under
aqueous conditions with colloidal alumina or a polyvalent metal
salt; curing the polymer to water insolubility; and, when required
to render the cured film water wettable, hydrolyzing the surface
portion only of the cured polymer.
Inventors: |
Rickert, Jr.; Milton E.
(Louisville, KY) |
Assignee: |
General Electric Company
(Louisville, KY)
|
Family
ID: |
25398183 |
Appl.
No.: |
05/891,438 |
Filed: |
March 29, 1978 |
Current U.S.
Class: |
204/488; 204/484;
427/341; 427/388.2; 427/409; 428/457; 428/460; 428/461; 62/515 |
Current CPC
Class: |
C25D
13/22 (20130101); Y10T 428/31688 (20150401); Y10T
428/31692 (20150401); Y10T 428/31678 (20150401) |
Current International
Class: |
C25D
13/22 (20060101); C23D 013/00 (); C25D 013/06 ();
F25B 039/02 () |
Field of
Search: |
;204/181R,181T,181C
;427/409,388A ;62/515 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Williams; Howard S.
Attorney, Agent or Firm: Millen & White
Claims
What is claimed is:
1. A process for rendering permanently water wettable a surface of
an article of manufacture which normally is not water wettable or
which loses its water wettability upon exposure to air or water,
which comprises the steps of:
(a) coating the surface with a continuous film of the free acid
form of a film forming acidic polymer which forms water soluble
salts and which is curable to water insolubility, said film
containing a curing agent for the polymer;
(b) contacting the thus-coated surface, under aqueous conditions
which retain the polymer coating on the surface, with colloidal
alumina or a water soluble salt of a polyvalent metal;
(c) curing the thus-contacted polymer on the substrate surface to
water insolubility; and, if required to render the cured polymer
coating water wettable, thereafter hydrolyzing substantially only
the surface portion of the cured polymer.
2. The process of claim 1 wherein the surface is metal.
3. A metal article of manufacture presenting a surface rendered
permanently water wettable by a water-insoluble coating thereon of
a water-soluble acidic polymer which has been cured to
water-insolubility and rendered water wettable according to the
process of claim 2.
4. The process of claim 2 wherein the metal is aluminum.
5. The process of claim 4 wherein the article of manufacture is a
refrigerator evaporator.
6. An article of manufacture formed of aluminum presenting a
surface rendered permanently water wettable by a water-insoluble
coating thereon of a water-soluble acidic polymer which has been
cured to water-insolubility and rendered water wettable according
to the process of claim 4.
7. An aluminum article of manufacture according to claim 6 wherein
the polymer is a polyacrylic acid.
8. An aluminum refrigerator evaporator according to claim 7.
9. A process of claim 1 wherein the polymer and the curing agent
are applied to the surface from an aqueous solution of a mixture of
the curing agent and an ammonium or amine salt of the polymer.
10. A process of claim 9 wherein the surface is a metal and the
surface is coated with the polymer in free acid form by anodic
electrodeposition.
11. The process of claim 9 wherein the surface is coated with the
ammonium or amine salt of the polymer and the polymer in the
coating is then converted to its free acid form prior to step (b)
by heating the coating below the curing temperature of the
polymer.
12. The process of claim 1 wherein the curing agent for the polymer
is a cross-linking agent for the polymer under the influence of
heat and the polymer is cured by heating.
13. The process of claim 12 wherein the cross-linking agent is a
melamine-formaldehyde condensate.
14. The process of claim 1 wherein the polymer is a polyacrylic
acid.
15. The process of claim 1 wherein in step (b) the coated surface
is contacted with an acidic aqueous colloidal dispersion of
alumina.
16. The process of claim 1 wherein in step (b) the coated surface
is contacted with an acidic aqueous solution of salt of magnesium,
aluminum, calcium, manganese, iron, cobalt, nickel, copper or
zinc.
17. The process of claim 16 wherein the salt is a manganous
salt.
18. The process of claim 16 wherein the salt is a magnesium
salt.
19. The process of claim 16 wherein the salt is a calcium salt.
20. The process of claim 16 wherein the salt is an aluminum
salt.
21. The process of claim 1 wherein in step (c) the polymer is
hydrolyzed with water at a pH of 7 or higher.
22. The process of claim 1 wherein the polymer coating has a
thickness of about 3.times.10.sup.-3 to about 40.times.10.sup.-3
mm.
23. The process of claim 1 wherein in step (b) the coated surface
is immersed briefly in the aqueous dispersion or solution.
24. The process of claim 1 wherein the surface is aluminum, wherein
the curing agent is a cross-linking agent for the polymer under the
influence of heat, wherein the acidic polymer is a polyacrylic acid
which is coated on the surface in step (a) from an aqueous solution
of an ammonium or amine salt thereof, and wherein in step (c) the
polymer is cured by heating.
25. The process of claim 24 wherein in step (a) the surface is
coated with the polymer in free acid form by anodic
electrodeposition.
26. The process of claim 25 wherein in step (b) the coated
substrate is immersed in an acidic aqueous colloidal dispersion of
alumina or with an acidic aqueous solution of the polyvalent metal
salt while the substrate is a cathode in a field of direct
electrical current passed through the suspension or solution,
thereby forming a coating on the substrate which is permanently
water wettable upon curing.
27. The process according to claim 26 wherein in step (b) the
coated substrate is connected to the negative terminal of a source
of direct current and is immersed into an acidic aqueous colloidal
dispersion of alumina, through which the direct current is
passed.
28. The process of claim 24 wherein the polymer coating has a
thickness of about 3.times.10.sup.-3 to about 40.times.10.sup.-3
mm.
29. The process of claim 28 wherein the article of manufacture is a
refrigerator evaporator, wherein in step (a) the surface is coated
with the polymer in free acid form by anodic electrodeposition, and
wherein in step (b) the coated surface is immersed briefly in an
acidic aqueous colloidal dispersion of alumina.
30. The process of claim 28 wherein the article of manufacture is a
refrigerator evaporator, wherein in step (a) the surface is coated
with the polymer in free acid form by anodic electrodeposition, and
wherein in step (b) the coated surface thereof is immersed briefly
in an acidic aqueous solution of a magnesium, aluminum, calcium or
manganous salt.
31. The process of claim 24 wherein in step (b) the coated surface
is immersed briefly in an acidic aqueous solution of a salt of Mg,
Al, Ca, Mn, Fe, Co, Ni, Cu or Zn.
32. The process of claim 31 wherein the metal salt is a magnesium
aluminum, calcium or manganous salt.
33. The process of claim 24 wherein the curing agent for the
polymer is a melamine formaldehyde condensate.
34. The process of claim 24 wherein in step (b) the coated surface
is immersed briefly in an acidic aqueous colloidal dispersion of
alumina.
35. The process of claim 24 wherein the article of manufacture is a
refrigerator evaporator.
36. A process according to claim 1 wherein in Step (c) the coated
surface is rendered water wettable by hydrolyzing substantially
only the surface portion of the cured polymer.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for rendering surfaces
permanently water wettable and to water wettable articles of
manufacture thus-produced.
It is sometimes desirable that a surface of an article of
manufacture exposed to water be water wettable. The term "water
wettable" as used herein means the ability of the surface to retain
a substantially unbroken film of water thereon, preferably for at
least 10 seconds and more preferably for at least 30 seconds after
removal from water, e.g., after being immersed in water or flushed
with a stream of water. This property is desirable because a water
wettable surface of an article of manufacture exposed to water
condensation has less of a tendency to drip droplets of water
therefrom than a surface which is not water wettable. Instead of
dripping in a random fashion from the surface, the water migrates
downwardly along the surface in a predictable path and if droplets
form, they can be collected at a predetermined desired point. An
example of articles of manufacture where water wettability is a
necessary property is an aluminum refrigerator evaporator, whose
external surfaces conventionally are rendered permanently water
wettable by metal anodizing, and the external surfaces of other
evaporators and heat exchangers whose efficiency is dependent on
the surfaces being wet with water. Other examples of articles of
manufacture in which water wettability is a desirable property are
those which are maintained below ambient temperature in a humid
atmosphere and which are therefore susceptible to condensation and
dripping, e.g., plastic, for example, PVC, polyethylene, and metal,
for example, aluminum, copper and iron, cold water pipes and sheets
forming the interior surfaces of refrigerators.
Another example of articles of manufacture where water wettability
is a desirable or necessary property are those formed of or having
a coating on the surface thereof of a hydrophobic polymer, which
are to be coated with an aqueous coating composition. It is
difficult and sometimes impossible to apply a uniform coating of a
desired aqueous coating composition to such hydrophobic surfaces.
Therefore, the surface of the polymer must be physically or
chemically altered to render it receptive to the selected aqueous
coating composition.
It is relatively simple to render metal surfaces temporarily water
wettable. For example, acidic aqueous colloidal dispersions of
.alpha.-alumina are used commercially to render surfaces
temporarily water wettable. See Trade Literature "Dispal.RTM.
Alumina", Continental Oil Company, Market Development Dept., Saddle
Brook, N.J. Although that literature states that the durability of
the alumina film is increased by incorporating a small percentage,
e.g., 2%, of acrylic emulsion in the dispersion, the wettability
imparted by the alumina film is still relatively impermanent and
moreover, is far less effective than anodizing in protecting the
aluminum substrate from corrosion and/or staining. Therefore, this
method of rendering aluminum articles of manufacture water wettable
is unsatisfactory for products which are subjected for long periods
of time to conditions which could stain or corrode the surface of
the aluminum, as is the case of the interior aluminum surfaces of
refrigerator boxes and the exterior surfaces of refrigerator and
air conditioner evaporators.
The coating of aluminum with polymeric coatings is, of course, well
known in the prior art. U.S. Pat. No. 3,464,854 discloses a coating
composition which can be applied to aluminum surfaces, consisting
of an eposy resin binder in aqueous dispersion with a major amount
of particulate filler, e.g., alumina. Additionally, an amine curing
agent is provided in the composition. U.S. Pat. No. 3,468,753
describes a method for applying a layer of co-polymer ethylene and
unsaturated carboxylic acids such as acrylic and methacrylic acid,
which layer may include a filler material, to a metal sheet, e.g.,
aluminum.
The electrodeposition of acidic polymers on metal substrates is
well known. See, e.g., U.S. Pat. Nos. 3,230,162; 3,463,714;
3,494,847; 3,507,765; 3,532,613; 3,556,972; 3,575,902; 3,741,923;
and 3,755,119.
The hydrophilization of hydrophobic polymers is also known in the
prior art. U.S. Pat. No. 3,935,342 discloses a process wherein
hydrophobic polymers are treated by contacting them with silylated
acrylate or methacrylate monomers, polymerizing the monomer and
then hydrolyzing the polymer to break the siloxy bonds and produce
free hydroxy groups. The product can be used in a large number of
coating applications. See, Col. 1, lines 45-53.
It is an object of this invention to provide a process for
rendering non-polar or hydrophobic surfaces permanently water
wettable. Another object is the provision of novel articles of
manufacture having a surface rendered permanently water wettable
according to the process of this invention. Other objects will be
apparent to those skilled in the art to which this invention
pertains.
SUMMARY OF THE INVENTION
In a process aspect, this invention relates to a process for
rendering permanently water wettable a surface of an article of
manufacture which normally is not permanently water wettable, which
comprises the steps of:
(a) coating the surface with a continuous coherent film of the free
acid form of a film forming acidic polymer which forms water
soluble salts and which is curable to water insolubility, said film
containing a curing agent for the polymer;
(b) contacting the thus-coated surface, under acidic aqueous
conditions which retain the polymer coating on the surface, with
colloidal alumina or a water soluble polyvalent metal salt;
(c) curing the thus-contacted polymer on the surface to water
insolubility, and, when required to render the cured polymer water
wettable, thereafter hydrolyzing substantially only the surface
portion of the cured polymer.
In an article of manufacture aspect, this invention relates to
aluminum articles of manufacture formed of aluminum presenting a
surface rendered permanently water wettable by a water-insoluble
coating thereon of a water-soluble acidic polymer which has been
cured to water-insolubility and has been rendered water wettable
according to the process of this invention.
DETAILED DISCUSSION
The articles of manufacture whose surface can be rendered
permanently water wettable according to the process of this
invention are those formed from materials which normally are not
water wettable or which lose their water wettability after
manufacture, upon exposure to air and/or water and thus include
those formed from a metal, e.g., aluminum, copper, brass, bronze,
nickel, iron, stainless steel and noble metals, e.g., gold, silver
and platinum, and those formed from or coated with a water
insoluble hydrophobic polymer, including the thermoplastics, e.g.,
polystyrene, styrene-acrylonitrile resins, polyvinyl chloride,
polyvinyl acetate, polyethylene, polypropylene, polybutenes,
acetals, acrylics, thermoplastic polyesters and nylons, and the
thermoset polymers, e.g., alkyd, epoxy, phenolic, phenol-aralkyl,
urea and melamine resins and thermoset unsaturated polyesters and
polyurethanes. If the article of manufacture is formed of or coated
with a polymer, necessarily the polymer must be one which can be
coated with a water soluble acidic polymer as defined herein and
which is stable in the curing step. Preferred substrates are those
formed from a base metal, especially those formed from aluminum or
an aluminum alloy, e.g., of high Si content, or a composite of
aluminum and steel because, inter alia, the aluminum is protected
from the staining which occurs on unprotected aluminum, as well as
being rendered permanently water wettable.
The article of manufacture can be in any solid shaped form,
including rigid and flexible sheets, films, foils and extruded,
cast, stamped and machined three-dimensional shaped articles,
indcluding water pipes and the heat exchanger surfaces of air
conditioners and evaporators and the frames of refrigerator doors,
drain troughs, fresh food liner tops and other refrigerated areas
which are susceptible to sweating. Preferred are those forming the
heat exchanger surface of evaporators, especially aluminum
refrigerator evaporators, and other fabricated metal articles of
manufacture.
The acidic polymers used to coat the starting substrate are those
which:
(a) form water soluble salts;
(b) will form a continuous coherent film on the selected substrate;
and
(c) are curable to water-insolubility.
These properties, rather than the specific chemical structure of an
acidic polymer, determine its suitability for use in this
invention. Therefore, a wide variety of structural types of
film-forming acidic polyelectrolytes can be employed, including
homopolymers of acrylic and methacrylic acid having polymer units
of the formula ##STR1## wherein R is H or CH.sub.3 and n is an
integer, e.g., about 900 or higher, and copolymers with other
monomers which form soluble amine or ammonium salts, e.g., vinyl
alkyl ether-maleic acid copolymers. Acidic polymers in which the
acidic group is a sulfonic acid group, e.g., polystyrene sulfonic
acids and polyethylene-sulfonic acids, a sulfato group, e.g.,
partially sulfated polyvinyl alcohol, or a phosphonic acid group,
e.g., polyvinyl phosphonic acid, are also operable, as are
incompletely hydrolyzed alkyl acrylate and alkyl methacrylate
polymers, i.e., polymers having polymer units of the formula
##STR2## wherein R and n are as defined above and a portion of the
R.sub.1 groups are alkyl, e.g., methyl or ethyl, and a remainder
portion, sufficient to render ammonium and/or amine salts of the
polymer water soluble, are H. Preferred are the polymers of acrylic
acid. For a discussion of such polymers, see Encyclopedia of
Polymer Science and Technology, Vol. 10, pp. 781 et seq. (John
Wiley & Sons, Inc. 1969); Kirk-Othmer, Encyclopedia of Chem.
Tech., Vol. II, pp. 874 et seq.; Davidson & Sittig, Water
Soluble Resins, 2nd Ed., Chapter 8 (Reinhold Pub. 1968); D. H.
Solomon, The Chemistry of Organic Film Formers, Chapter 10 (John
Wiley & Sons, Inc., 1967).
The polymers are those which form water soluble salts with bases.
The term "water soluble" as used herein embraces "colloidally
dispersible in water", since many of the high molecular weight
polymers which are operable in the process of this invention form
colloidal dispersion in water rather than true solutions.
A preferred class of acidic polymers which are soluble or
colloidally dispersible in water are those which can be deposited
as a film on a metallic substrate by anodic electrodeposition,
e.g., a co-polymer of acrylic acid, methacrylic acid, crotonic
acid, .beta.-benzoyl-acrylic acid, fumaric or maleic acid and of an
alkyl acrylate, a styrene, an alkyl styrene, a vinyl monomer, e.g.,
vinyl chloride, vinyl acetate acrylamide, or a vinyl monomer having
an alcoholic hydroxy group, e.g., the hydroxyalkyl esters of
acrylic acid, alone or in combination with one of the other
monomers named above. Preferably, the acidic polymer has an
electrical equivalent weight between about 1,000 and 20,000 and an
acid number of at least 20, preferably about 30 to 300. Another
class is an at least partially neutralized reaction product of a
drying oil fatty acid ester with an .alpha.,.beta.-ethylenically
unsaturated dicarboxylic acid or an anhydride thereof, e.g., maleic
or itaconic anhydride, maelic, itaconic or fumaric acid. Still
another class is the polyamide acid reaction product of a
dianhydride, e.g., benzophenone dianhydride or pyromelletic
dianhydride, and a diamine, e.g., a C.sub.2-8 alkylenediamine or
m-phenylenediamine. Still another class is an at least partially
neutralized dicarboxylic acid of a block copolymer of
.alpha.-methylstyrene and an aliphatic conjugated diolefin of 3-6
carbon atoms. A further class is a polycarboxylic acid resin having
olefinically unsaturated side chain produced by first reacting a
polydioxylated polymer with an olefinically unsaturated acyclic
carboxylic acid of about 18 carbon atoms, reacting this product
with trimellitic anhydride and reacting this product with a
polyolefin glycol of a molecular weight greather than about 200.
Another class are the heat curable acrylate resins containing
50-90% by weight of an ester of acrylic and/or methacrylic acid and
a monoalcohol of 1-8 carbon atoms, 1-15% by weight of at least one
copolymerizable olefinically unsaturated carboxylic acid of 3-5
carbon atoms, e.g., itaconic acid, acrylic or methacrylic acid; and
5-49% by weight of a further copolymerizable olefinically
unsaturated compound, e.g., vinyl piralate, styrene acrylonitrile,
acrylamide and/or ethers of methylolacrylamide and methacrylamide,
e.g., N-butoxymethylmethacrylic acid amide and ethylene glycol
esters of acrylic acid. For a further description of such
electrodepositable acidic polymers, see U.S. Pat. Nos. 3,230,162;
3,463,714; 3,494,847; 3,507,765; 3,532,613; 3,556,972; 3,575,902;
3,741,923; and 3,755,119, whose disclosures are incorporated herein
by reference.
In addition to ammonia, a wide variety of amines can be employed to
render the polymer water soluble. Preferred are those
conventionally employed to form water soluble amine salts of
polymers, viz., low molecular weight aliphatic amines, preferably
tertiary alkylamines, alkanolamines and cycloaliphatic amines,
e.g., dialkylaminoalkanols. When the acidic polymer is deposited on
the substrate as an amine salt, the amine must be sufficiently
volatile to be removed when the coated substrate is dried. Examples
of such amines are dimethylaminoethanol, ethanolamine and
triethylamine.
Necessarily, the selected polymer must be one which is capable of
forming a continuous film on the starting substrate. The preferred
acrylic acid polymers will form such films on many substrates.
However, with some polymers and/or hydrophobic substrates, some
adjustment of the vehicle from which the polymer is deposited,
e.g., by varying the concentration of the polymer therein or adding
a wetting agent and/or an organic solvent thereto, may be required.
However, such techniques are standard and will known in the coating
art.
The film of the selected acidic polymer deposited on the selected
substrate must also be curable to water insolubility both at acid
and alkaline pH's. In other words, the cured polymer is neither
water soluble nor forms water soluble salts. This is accomplished
by including a curing agent in the film deposited on the selected
substrate. Such curing agents conventionally are cross-linking
agents for the polymers and which effect cross-linking when the
film is heated or is exposed to another form of energy required to
initiate the cross-linking reaction, e.g., actinic or ultraviolet
light or gamma ray irradiation. In addition to the activating
energy, a reaction catalyst is sometimes also required. To the
extent one is required, e.g., hydrochloric or p-toluenesulfonic
acid, it is included in the term "curing agent" as used herein.
An example of a class of curing agents for acidic polymers are
water soluble aminoplasts and their precursors which cross-link the
acidic polymers. Aminoplasts are a conventional component in
coating compositions. Many of these have the formula --NH--CH.sub.2
--OR wherein R is hydrogen or alkyl of one to four carbon atoms and
the unsatisfied valence is an organic moiety. Examples are the
condensation products of aldehydes, particularly formaldehyde, with
several amino- or amido-group-carrying substances, such as, for
example, with melamine, urea, N,N'-ethyleneurea, dicyanodiamide,
and benzoguanamine. There can also be used water soluble polymers
having the structure of copolymerizates into which is polymerized
an amide of an .alpha.-ethylenically unsaturated carboxylic acid
having N-methylol- and/or N-methylol-ether groups.
Aminoplasts which are modified with alcohols, preferably alkanols
of one to four carbon atoms, can also be used. In place of these
resinous products, it is preferable to employ precursors of
aminoplasts, e.g., dimethylol urea, tetramethylol benzoguanamine,
trimethylol melamine or hexamethylol melamine, which can also be
employed in a partially or completely etherified form, for example,
as dimethoxymethyl urea, tetrakis(methoxymethyl)benzoquanamine,
tetrakis(ethoxymethyl)benzoguanamine, or polyethers of hexamethylol
melamine, such as hexakis(methoxymethyl)melamine or
hexakis(butoxymethyl)melamine. It is also possible to use mixtures
of all the above N-methylol products.
Thus, a wide variety of commercially available aminoplasts can be
used for combining with the special polyesters of the present
invention. For more details regarding the aminoplasts which can be
used, see "Organic Protective Coatings," Von Fischer and Bobaleck,
1953, Reinhold, pages 210-255.
In step (b) of the process of this invention, the free acid form of
the acidic polymer forming the coating on the substrate is
contacted under aqueous conditions with colloidal alumina or a
water soluble salt of a polyvalent metal, i.e., a metal having a
valence greater than one.
Of the salts of polyvalent metals, preferred are salts of metals of
the Group IB, IIA, IIB, IIIA, VIIB or VIII series, more preferably
of Mg, Al, Ca, Mn, Fe, Co, Ni, or Zn. Of these, the aluminum and
divalent metal salts, e.g., magnesium, calcium and manganous salts
are especially preferred. The cation can be that of any organic or
inorganic acid or acidic compound which forms a soluble salt with
the selected metal, including organic carboxylic acids, e.g.,
acetic; organic sulfonic acids, e.g., benzenesulfonic and
p-toluenesulfonic; and inorganic hydrohalic acids and acids of
sulfur and phosphorous, e.g., hydrochloric, sulfuric and phosphoric
acids.
In carrying out Step (a) of the process of this invention, a
surface of the selected starting substrate is coated with a
continuous film of the free acid form of the selected acidic
polymer containing a curing agent for the polymer. This can be
accomplished in one embodiment by dipping the surface of the
substrate in or spraying the surface of the substrate with an
aqueous solution of a mixture of the ammonium salt or a volatile
amine salt of the acidic polymer and curing agent and then drying
the film below curing temperature to drive off free ammonia or
amine from the film. The drying step is ordinarily required to
render the surface portion of the polymer film receptive to the
colloidal alumina or polyvalent metal salt employed to render the
polymer coating water wettable. presumably, this enhanced
receptiveness of the polymer is due to partial conversion of the
polymer to the free acid form by the liberation of part of the
amine or ammonia thereof during the drying step, since a drying
step is not required when the polymer coating is applied
electrolytically, as described hereinafter. Apparently, a drying
step is not required because the polymer is deposited
electrolytically on the substrate substantially in its free acid
form, rather than as an amine or ammonium salt thereof when the
polymer is applied by immersing the substrate in or spraying it
with a solution of the ammonium or amine salt of the acidic
polymer.
The film of the acidic polymer is applied electrolytically to the
substrate by employing the substrate as an anode and the aqueous
solution of the ammonium or amine salt of the acidic polymer as the
electrolyte in an electrolysis system. For a description of
techniques for doing so on a commercial scale, see "William
Brushwell Reports: Coatings, Update," American Paint & Coatings
Journal, pp. 65-70, Oct. 3, 1977; Loyd R. Brower, Jr. Standard T
Chemical Co., Inc., Technical Bulletin, "Electro-Painting
Principles and Process Variables." For a comparison of anodic and
cathodic electrodeposition processes, see Wismar and Busso, "Make
the Part the Cathode: Key to Resistant Coatings," Chem. Eng. pp.
115-118, June 14, 1971, and references cited therein.
To deposit the polymer electrolytically, necessarily the substrate
must be electroconducting. Therefore, for all practical purposes,
this technique is limited to metal substrates. Of these, aluminum
is preferred. Typically, the substrate to be coated is electrically
connected to the positive terminal of a source of direct current
and the portion of the substrate to be coated with the polymer is
immersed in an aqueous solution of a salt of the polymer, along
with a cathode connected to the negative terminal source of direct
current. A voltage is then imposed on the solution which produces
the amperage required to deposit the polymer. As is well known in
this art, this coating is self-limiting because of the reduction in
electroconductivity of the substrate resulting from the polymer
film deposited thereon.
The acidic polymer is preferably dissolved in the form of an
ammonium or amine salt thereof. Although metal, e.g., Na or K,
salts are also operable, they are not preferred because a portion
of the solubilizing cation is occluded in the deposited film and an
acid washing step way be required to remove enough metal ion to
render the polymer receptive to the colloidal alumina or polyvalent
metal salt, whereas such a step ordinarily is not required when the
ammonium or an amine salt is employed.
A film of the free acid form of the polymer can also be applied to
the substrate by coating the substrate with a solution of the free
acid form of the polymer in an organic solvent and then
volatilizing the organic solvent, or by electrostatic spraying or
hot melt techniques, employing conditions which leave a continuous
film of the free acid form of the polymer on the substrate. Such
conditions are well known in the coating art.
Preferably, the polymer coating applied to the substrate has a
thickness of about 3.times.10.sup.-3 to about 40.times.10.sup.-3
mm., more preferably about 9.times.10.sup.-3 mm. The thickness of
the coating can be regulated by the concentration of the polymer in
the solution, the viscosity of the solution, the number of
immersions, the spraying time and/or the final voltage when the
coating is applied electrolytically.
After the coating has been applied, it is sometimes desirable to
wash the coating with water or acid to remove extraneous anions
therefrom or to convert the polymer more completely to its free
acid form. As stated above, a heating step is required, if the
polymer is deposited as its ammonium or amine salt, to volatilize
the ammonia or amine, thereby converting the polymer to its free
acid form.
In step (b) of the process of this invention, the free acid form of
the acidic polymer forming the film coating on the substrate is
contacted with colloidal alumina or with a water soluble salt of a
polyvalent metal under aqueous conditions, preferably at a pH of
less than 7, e.g., about 3 to 5.
The acidic polymer forming the coating on the substrate can be
hydrated or dry when contacted with the colloidal alumina or the
divalent metal salt under aqueous conditions. If the polymer is
moist, the colloidal alumina or polyvalent metal salt can be
applied as a dry solid, preferably a micronized powder, with the
moisture of the coating providing the requisite aqueous vehical for
the interaction of the alumina or metal salt with the acidic
polymer. However, it is ordinarily more convenient and preferred to
employ an aqueous suspension of the alumina or aqueous solution of
the metal salt.
Since the interaction thereof with the acidic polymer occurs
predominantly only on the surface of the polymer coating, the
theoretical minimum amount of alumina or metal salt required to
effect the requisite interaction with the acidic polymer is
miniscule. This theoretical minimum can be calculated from the
surface area of the polymer coating and the number of acid groups
thereon. However, it is not necessary to do so because the most
convenient method of contacting the acidic polymer with the alumina
or metal salt is by immersing the coated substrate in an aqueous
colloidal suspension of the alumina or aqueous solution of the
metal salt, thereby contacting the acidic polymer with an amount of
alumina or metal salt far in excess of the theoretical minimum.
The aqueous colloidal suspension of the alumina and the aqueous
solution of the selected polyvalent metal salt are ordinarily
acidic. However, to the extent a neutral colloidal suspension of
the alumina or solution of the metal salt can be formed, they also
can be employed, so long as the polymer is in its free acid form
when contacted therewith.
In step (c) of the process of this invention, the polymer coating
on the substrate is rendered permanent by curing the polymer to
water insolubility, i.e., both the free acid form of the polymer
and salts thereof are insoluble in water. The conditions employed
in the curing step are determined by the selected polymer and
curing agent therefor. Although curing of the acidic polymer can be
achieved with some curing agents and acidic polymers by irridiation
with actinic, or ultraviolet light or gamma rays, it is ordinarily
preferred to cure by heating the coated substrate, e.g., at about
135.degree. to 250.degree. C., preferably about
150.degree.-200.degree. C., until the acidic polymer is
insolubilized, usually for a few minutes, e.g., from about 30
seconds to one or more hours, preferably about 5 to 30 minutes.
Methods for determining the optimum conditions of time and
temperature are well known in the coating art.
The cured polymer ordinarily is not water wettable and therefore
hydrolysis of the surface portion thereof is required to impart
water wettability thereto. Only when the acidic polymer is
contacted with the colloidal alumina or the polyvalent metal salt
under conditions of cathodic electrodeposition is the polymer
coating water wettable after curing without the necessity of a
subsequent hydrolysis step. However, only a mild hydrolysis at any
pH is required, e.g., water at 70.degree. to 100.degree. C. for
about 30 to 5 minutes; dilute aqueous NH.sub.4 OH or NaOH, e.g.,
0.25-0.05 M, at 20.degree. to 70.degree. C. for about 10 minutes to
10 seconds; or dilute acetic or sulfuric acid, e.g., 0.01 to 0.1 M,
at 20.degree. to 70.degree. C. for about 10 to 1 minutes.
The cured and, when required, also surface hydrolyzed, polymer
coating on the substrate is permanent as evidenced by its
resistance to abrasion, i.e., 10 finger strokes with a wet paper
towel, and resistance to water, i.e., 10 days immersion at
49.degree. C. It is water wettable, as evidenced by the retention
of a substantially continuous film of water thereon while in a
vertical position for at least 10 seconds after immersion in or
flooding with water. It is permanently water wettable, as evidenced
by its ability to retain its water wettability after being immersed
for 240 hours in water at 49.degree. C.
Without further elaboration, it is believed that one skilled in the
art can, using the preceding description, utilize the present
invention to its fullest extent. The following preferred specific
embodiments are, therefore, to be construed as merely illustrative
and not limitative of the remainder of the disclosure in any way
whatsoever.
EXAMPLE 1
50.9 grams of hexamethoxymethylmelamine (American Cyanamide,
Cymel.RTM. 303) was added to 204.6 grams of Celanese ED 25166
acrylic acid resin (acid value 80.+-.3). The mixture was blended
thoroughly with a spatula. To this mixture was added 17.0 grams of
2-dimethylethanolamine. The mixture was blended with a spatula and
while stirring, 527.5 grams of deionized water was added very
slowly, until a uniform solution was obtained. This solution was
diluted by adding an additional 800 milliliters of deionized
water.
A clean aluminum and a clean stainless steel panel were partially
immersed in the above solution. The exposed portions of the
aluminum and stainless steel panels were electrically wired to the
positive and negative output terminals respectively of a direct
current rectifier, such that when energized, the aluminum panel
would have a positive potential with respect to the stainless steel
panel. The voltage was increased from zero to 25 volts while
maintaining current flow at about one ampere. When the applied
voltage reached 25 volts, the voltage was held constant for 10
minutes. The aluminum panel was separated from the rectifier and
the coating deposited on the aluminum panel was rinsed with
deionized water.
The rinsed panel was then dipped for three seconds in a colloidal
dispersion of hydrated aluminum oxide prepared by adding 19 grams
of glacial acetic acid to 877 grams of deionized water in a General
Electric Model BL3 blender; with the blender on stir, slowly adding
104 grams of Conoco "Catapal".RTM. SB Alumina to the dilute acid;
stirring the mixture for one hour; and then diluting the mixture
with an equal volume of deionized water.
The coated panel was then heated in a forced air oven for 20
minutes at 182.degree. C. The panel was allowed to cool to
approximately 65.degree. C. and the coated portion of the panel was
immersed in tap water for 15 minutes at 93.degree. C.
The coated panel was wetted under running tap water and upon
removal from the running stream, the water sheeted or spread only
on the portion of the panel which had received the 93.degree. C.
water treatment. The sheet was retained for more than 10 seconds
with the panel in a vertical position. The contact angle of
deionized water on the wettable portion of the panel was
53.degree.-56.degree.. The coating was abrasion resistant requiring
51 cycles with an RCA tape abrader (273 gram load) before wearing
through the approximately 5-7.5.times.10.sup.-3 mm. coating. The
panel was immersed in 49.degree. C. water for 4 days. The uncoated
portion of the panel turned very dark as a result of this exposure
but the coated portion of the aluminum appeared to be in
as-produced condition.
EXAMPLE 2
An aqueous coating solution of acrylic acid polymer and curing
agent was prepared by blending with a spatula 23.8 grams of
hexamethoxymethylmelamine (American Cyanamide Cymel.RTM. 303) and
8.2 grams of 2-dimethylethanolamine with 99.0 grams of Celanese
ED25166 acrylic acid resin. Next, a mixture of 4.7 grams of
Cellosolve, 0.8 grams of Cymel 303 and 0.5 grams of American
Cyanamide 4040 Acid Catalyst were spatula blended and then added to
the above mixture and 250 grams of deionized water was added very
slowly with stirring until a uniform solution was obtained. This
solution was then diluted by adding thereto an equal volume of
deionized water.
A clean aluminum panel was dipped in the resulting solution for
approximately 3 seconds and after withdrawal was heated for 5
minutes at 121.degree. C. The panel was cooled with deionized water
and then immersed for approximately 3 seconds in a colloidal
dispersion of alumina prepared as described in Example 1, to which
was then added to 200 ml. thereof, 1.0 grams of Rohm and Haas
Triton CF 54 and 0.5 grams of Wyandotte Chemical Corporation
Pluronic L61 wetting agents. The coated panel was then heated in a
forced air oven for 20 minutes at 178.degree. C. The panel was then
wetted under running tap water. Upon removal, the water formed a
continuous sheet on the coated panel which persisted with the panel
positioned vertically for about 10 sec. The coating is permanent,
as evidenced by inertness to water immersion (for 11 days at
49.degree. C.) and resistance to abrasion with a wet paper
towel--10 finger strokes.
EXAMPLE 3
Electrolytically coat a clean surface of an aluminum substrate with
an about 9 mm. coating of polyacrylic acid according to the
procedure of Example 1. After rinsing with deionized water, dip the
coated substrate in a 0.06 M (pH about 4) solution of magnesium
sulfate in deionized water. Without rinsing, cure the coating on
the substrate to water insolubility by heating in a forced air oven
at 182.degree. C. for 20 minutes and then hydrolyze the surface
portion only of the cured polymer by immersing the coated substrate
in hot (93.degree. C.) water for about 15 minutes, thereby
rendering the coated substrate water wettable.
Comparable results are obtained by substituting a like solution of
manganous sulfate or calcium acetate for the magnesium sulfate.
Other acidic solutions of soluble salts of polyvalent metals, e.g.,
aluminum acetate, zinc sulfate, cuprous sulfate, cobalt sulfate and
nickel sulfate, can also be substituted for the magnesium
sulfate.
EXAMPLE 4
Follow the procedure of Example 1, but hydrolyze the coated
substrate with 0.25 M NaOH at 60.degree. C. for 15 sec. or at room
temperature for 1-2 min., 0.05 M H.sub.2 SO.sub.4 at 38.degree. C.
for 5 min., 0.05 M acetic acid at 60.degree. C. for 5 min., or 0.05
M NH.sub.4 OH at room temperature for 2 min.
EXAMPLE 5
Follow the procedure of Example 1 except use 100 volts applied for
1 minute. Then immerse the acrylic acid polymer coated panel, after
rinsing with deionized water, and the stainless steel panel in the
colloidal alumina dispersion, connect them as negative and positive
output terminals, respectively, to a direct current rectifier and
apply a 75 volt potential for 1 minute. A second coating of alumina
is deposited on the coated panel. Without rinsing, heat the coated
aluminum panel in a forced air oven for 20 minutes at 178.degree.
C. The resulting coated aluminum panel is water wettable, having a
water contact angle of 51.degree.-61.degree. and is permanent as
evidenced by inertness to water immersion for 10 days at 49.degree.
C.
EXAMPLE 6
A preferred method of performing the process of this invention on a
commercial scale is shown schematically in the drawing.
A fresh food evaporator for a Model TB14 General Electric home
refrigerator from the fabricating plant is immersed in a cleaner
(Chemetron Alkalume.RTM. Ac 8) for one min. at 77.degree. C., then
sprayed with a water rinse for 30 sec. at 60.degree. C., then
sprayed with recirculating deionized water for 30 sec. at room
temperature and then for 5 seconds with fresh deionized water at
room temperature.
The washed evaporator is then connected to the positive terminal of
a direct current rectifier, the negative terminal of which is
connected to a stainless steel cathode immersed in a 120 gal.
recirculating bath of deionized water containing (by weight) 12.8%
Celanese ED 25166 brand polyacrylic acid resin (75% solids by
weight) with an acid value of 80.+-.3, 3.2% Cymel 303 (American
Cyanamid) brand of hexamethoxymelamine and 1.1% of
dimethylethanolamine. The evaporator is immersed in the bath for
one minute while passing a direct current (100 volts, average draw
1 amp/ft.sup.2) through the bath, thereby depositing a uniform
continuous coherent film (about 9.times.10.sup.-3 mm. thick) of the
polymer containing the methoxymelamine and dimethylethanolamine on
the exposed surfaces of the evaporator.
The coated evaporator is removed from the bath and rectifier and
washed for 30 seconds with deionized water. The washed evaporator
is then immersed at room temperature for 30 seconds either in (a)
130 gallons of the acetic acid acidified, stirred aqueous
suspension of colloidal alumina described in Example 1, or (b) 130
gallons of a 0.06 molar solution of magnesium sulfate in deionized
water.
Without rinsing, the coated evaporator is then transferred to a
circulating hot air (177.degree. C.) for 20 minutes. The evaporator
is then dipped for 15 seconds in 0.25 M NaOH at 60.degree. C. and
then spray rinsed with water at room temperature for one minute.
The thus-treated evaporator when wet with water forms a
substantially continuous film of water on its surface which
persists for at least 10 sec.
The preceding examples can be repeated with similar success by
substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
From the foregoing description, one skilled in the art can easily
ascertain the essential characteristics of this invention, and
without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various usages and conditions.
* * * * *